Studies in this laboratory have determined that a large dose of ovalbumin (100 mg) injected i.v. two days earlier directly influenced small dense thymocytes to undergo blast transformation, followed by a massive emigration of large low density cells from the thymus cortex to the splenic marginal zone-red pulp. This antigen induced migratory pathway is superimposed upon a normal pathway taken by far fewer large low density cortical thymocytes to these sites. This newly described pathway is independent of that taken by small dense thymocytes to the periarteriolar lymphoid sheath (classical thymus dependent area) which is apparently uninfluenced by antigen. Preliminary data indicate that in rats the Thy 1. surface marker characteristic of bone marrow cells and cortical thymocytes, but not medullary thymocytes or any peripheral lymphocytes, appears only on large cells present in the marginal zone-red pulp after injection of the suppressive dose of antigen mentioned above. The proposed studies will attempt to establish whether the cells emigrating from the thymus cortex in accelerated fashion after a large i.v. dose of antigen and subsequently found in marginal zone-red pulp of spleen: (1) bear the Thy 1. surface marker and (2) are destined to function as obligatory suppressor cells in the periphery. Both the nonspecific and specific aspects of suppressor cell phenomena will be tested.